1. Field of the Art
The present invention relates generally to an electrochemical element, and more particularly to an element including a multi-layer electrochemical cell which comprises a solid electrolyte body of zirconia ceramics.
2. Related Art Statement
There have been known various electrochemical devices, each of which includes an electrochemical element using solid electrolyte. These electrochemical devices are used, for example, as oxygen sensors to detect the oxygen concentration of an exhaust gas emitted from internal combustion engines of automotive vehicles. Typical examples of such oxygen sensors include an oxygen sensor which uses zirconia ceramics as an oxygen-ion conductive solid electrolyte material. Such an oxygen sensor operates to determine the oxygen concentration according to the principle of an oxygen concentration cell. Also known in the art are electrochemical devices or elements such as sensing and pumping elements for hydrogen, nitrogen, carbon dioxide, etc. In recent years, there has been an increasing trend that an electrochemical element used in such devices as indicated above incorporates an electrochemical cell of a laminar structure which comprises a layer of zirconia ceramics solid electrolyte, and an electrode or electrodes disposed in contact with a surface or surfaces of the zirconia ceramic layer.
In such an electrochemical element of a laminar or multi-layer type, a porous diffusion layer having a predetermined diffusion resistance is formed on a zirconia solid electrolyte layer, so as to cover an electrode disposed on the zirconia solid electrolyte layer, in order to control an atmosphere of a measurement gas to which the electrode is exposed for contact therewith. Such a diffusion layer is also assigned to function as a solid electrolyte layer through which a pumping current is caused to flow between a pair of electrodes, which are provided on opposite sides of the solid electrolyte layer. Further, the electrochemical element may include a porous protective layer for protecting the electrode on the zirconia solid electrolyte layer from direct exposure to the measurement gas.
Also, it is known that the solid electrolyte layer of an electrochemical cell is formed of partially-stabilized zirconia ceramic having a tetragonal crystal phase or a monoclinic crystal phase, to improve thermal-shock resistance characteristics of the solid electrolyte layer. In the case where the solid electrolyte layer of partially-stablized zirconia ceramic is used in combination with a porous diffusion or protective layer or a porous diffusion-solid electrolyte layer (diffusion layer also serving as a solid electrolyte layer) as previously indicated, such a porous layer suffers severe changes in mechanical strength and electrical properties during its use, if the porous layer is made of partially-stabilized zirconia ceramic of the same composition as that used for the solid electrolyte layer. The changes are severe, particularly when a porous diffusion layer is formed of a partially-stabilized zirconia ceramic material. In this case, micro-cracks appearing on the surface of the partially-stabilized porous zirconia ceramic will develop into a serious problem, viz., internal destruction or collapse of the porous structure of the diffusion layer, which results in changing the diffusion resistance and thereby degrading the accuracy of measurement of the electrochemical element.